The present invention relates to a cross-linkable or cross-linked rubber composition which is usable to constitute a tire tread and which has, in the non-cross-linked state, an improved reinforcement properties, and also to a process for the preparation of this improved processing ability and, in the cross-linked state, minimized hysteresis losses and composition. The invention also relates to a tire tread and a tire incorporating it.
Since fuel economies and the need to preserve the environment have become priorities, it has become desirable to produce mixes having good mechanical properties and as low a hysteresis as possible so that they can be processed in the form of rubber compositions usable for the manufacture of various semi-finished products involved in the constitution of tires, such as, for example, underlayers, sidewalls or treads, and in order to obtain tires having reduced rolling resistance.
To achieve such an objective, numerous solutions have been proposed, consisting in particular of modifying the structure of the diene polymers and copolymers at the end of polymerisation by means of functionalizing, coupling or starring agents. The very great majority of these solutions have concentrated on the use of functionalized polymers which are active with respect to carbon black, with the aim of obtaining a good interaction between the polymer thus modified and the carbon black.
By way of illustration of this prior art relating to reinforcing fillers formed of carbon black, mention may for example be made of US Patent Specification U.S. Pat. No. 3,135,716, which describes the reaction of living diene polymers at the chain end with a polyfunctional organic coupling agent in order to obtain polymers having improved properties. Mention may also be made of US Patent Specification U.S. Pat. No. 3,244,664, which discloses the use of tetra-alkoxysilanes as coupling agents or starring agents for diene polymers.
Silica has been used as reinforcing filler in cross-linkable rubber compositions, in particular those intended to constitute tire treads, for a long time. However, this use has remained very limited, owing to the unsatisfactory level of certain physical properties of such compositions, in particular abrasion resistance.
This is why it has been proposed, in order to overcome these drawbacks, to use functionalized diene polymers instead of the non-functionalized polymers which were used before, and in particular polymers functionalized by alkoxysilane derivatives, such as tetraethoxysilanes. For example, mention may be made of US Patent Specification U.S. Pat. No. 5,066,721, which describes a rubber composition comprising a diene polymer functionalized by an alkoxysilane having at least one non-hydrolysable alkoxyl radical, which makes it possible to eliminate the polymerisation solvent by steam stripping.
One disadvantage of these functionalization reactions lies in the coupling reactions which accompany them, which generally makes it necessary to use an excess of alkoxysilane and/or intensive mixing in order to minimize these coupling reactions.
Another drawback of these reactions lies in the later carrying out of the steam stripping operation, which is necessary to eliminate the polymerisation solvent.
In fact, generally, experience shows that the functionalized polymers obtained undergo changes in macrostructure during this stripping operation, which results in serious degradation of their properties, unless one is limited to using as functionalizing agent an alkoxysilane belonging to a restricted family, such as that described in the aforementioned document U.S. Pat. No. 5,066,721.
Consequently, it emerges from the above that the use of diene polymers comprising an alkoxysilane function to obtain rubber compositions comprising silica as reinforcing filler is not satisfactory, despite the improved physical properties of these compositions.
This is why research has been carried out on other functionalization reactions, always with a view to obtaining such rubber compositions. By way of example, mention may be made of French Patent Specification FR-A-2 740 778 in the name of the Applicant, which discloses the incorporation, in rubber compositions comprising as reinforcing filler silica in a majority proportion (for example comprising a blend of silica and carbon black), of diene polymers bearing at the chain end a silanol function or a polysiloxane block having a silanol end. For example, a functionalizing agent consisting of a cyclic polysiloxane is used, such as hexamethylcyclotrisiloxane. The functionalized polymers which are obtained can be separated from the reaction medium resulting in their formation by steam extraction of the solvent, without their macrostructure and, consequently, their physical properties, changing.
Mention may also be made of European Patent Specification EP-A-877 047, which discloses the incorporation of such polymers having a silanol function in rubber compositions comprising as reinforcing filler carbon black having silica fixed to its surface.
It has been established that these polymers impart rubber properties, in particular hysteresis and reinforcement properties in the cross-linked state, which are improved compared with those of “control” compositions based on non-functional diene polymers, and which are at least analogous to those of compositions based on diene polymers having an alkoxysilane function.
Mention may also be made of European Patent Specification EP-A-692 493, which establishes that diene polymers bearing at the chain end alkoxysilane groups and an epoxy group result in improved reinforcement properties and in reduced hysteresis losses at low and high deformations.
One disadvantage of these polymers, which comprise a functional group which is active for coupling to silica or to carbon black surface-modified by silica, is that the improvement in the hysteresis and reinforcement properties which they impart to the rubber compositions incorporating them is generally accompanied by a processing ability of the non-cross-linked mixes which is compromised compared with that of non-functional “control” polymers.
Among the other functionalization reactions studied, mention may be made of the functionalization of diene polymers prepared in solution along the chain by COOH functions.
This COOH functionalization along the chain can be effected by direct metallation, in the presence of N,N,N′,N′-tetramethylethylenediamine (TMED), by means of butyllithium or metallic sodium (as described in patent specifications U.S. Pat. No. 3,978,161 and U.S. Pat. No. 3,976,628, respectively), followed by a carbonation reaction by means of carbon dioxide.
Such a process has the disadvantage of generally resulting in breaks in the chain of the modified polymer.
For grafting COOH functions along the chain of a diene polymer by a post-polymerisation grafting reaction, there have also been used mercaptans comprising a carbonyl group, such as a carboxylic acid group (for example 3-mercaptopropionic acid), an aldehyde group or a primary alcohol group, and amine compounds comprising such a carbonyl group.
By way of examples of these mercaptans or amine compounds comprising a carbonyl group, mention may be made respectively of two reagents of respective formulae HSCH2CO2CH3 and N2CHCO2CH2CH3. For the description of the reaction mechanisms relating to the use of these two reagents, reference may be made to the articles “K. Sanui, R. W. Lenz, W. J. MacKnight, J. Poly. Sci., Polym. Chem. Ed. 12, 1965 (1974)” and “H. Tanaka, W. J. MacKnight, J. Poly. Sci., Polym. Chem. Ed. 17, 2975 (1979)”.
One major disadvantage of using these carbonylated mercaptans or amino-esters is that it always results in significant changes in macrostructure characterized by great degradation of the distribution of the molecular weights for the modified polymer, which makes these reagents undesirable within the context of the present invention.
The functionalization by COOH functions along the chain may also be effected by means of carbon monoxide, either by hydroformylation followed by oxidation of the aldehyde formed (as described in US Patent Specification U.S. Pat. No. 4,912,145), or by direct hydrocarboxylation of the polymer (as described in the article “A. Nait Ajjou, H. Alper, Macromolecules 29, 1784 (1996)”). The catalysts used for these reactions are based on rhodium or palladium.
One disadvantage of this functionalization by carbon monoxide lies, on one hand, in the drastic nature of the operating conditions and, on the other hand, in the frequent formation of a gel in the reaction medium.
Functionalization by means of maleic anhydride is more widespread. It makes it possible to obtain succinic anhydride units, which are precursors of the COOH functions, along the chain by post-polymerisation grafting. Reference may be made to US Patent Specification U.S. Pat. No. 4,082,817 for an example of implementation of such functionalization. However, this type of functionalization may also result in the formation of a gel.
Radical polymerisation in emulsion is also known to yield polymers having COOH functions along the chain. For a general description of this type of polymerisation, reference may be made to the work “Emulsion Polymerization and Emulsion Polymers”, P. A. LOVELL and M. S. EL-AASSER, John Wiley and Sons (1997), pp. 558-561 (see also the references cited therein).
In the past, attempts have been made to use diene elastomers having COOH functions along the chain to prepare rubber compositions usable in tires, as is illustrated by the documents commented on hereafter.
US Patent Specification U.S. Pat. No. 5,494,091 in fact discloses a rubber composition filled with carbon black comprising from 25 to 55 phr of polyisoprene and from 45 to 75 phr (phr: parts by weight per hundred parts of elastomeric matrix) of a diene polymer belonging to the group consisting of homopolymers of conjugated dienes and copolymers of conjugated dienes with mono-olefins, such as EPDM terpolymers (ethylene, propylene and a diene), part of this polymer comprising COOH functions grafted along the chain by reaction with an unsaturated metal carboxylic acid salt, for example zinc dimethacrylate. The composition obtained is supposed to have a sufficiently high rigidity to be used in an internal reinforcement rubber for tire sidewalls, so as to permit travel with a flat tire under satisfactory conditions.
Patent specification WO-A-01/14470 discloses a process for manufacturing a tire, consisting of cross-linking, during the curing of the tire, a cross-linkable rubber composition included in this tire based on a diene elastomer prepared in solution or in emulsion and comprising COOH functions along the chain, without using the slightest cross-linking system. This cross-linkable rubber composition comprises an epoxidised organic liquid which is intended to allow the cross-linking thereof upon curing.
These compositions may equally well comprise carbon black, silica or any other reinforcing filler known in the tire industry and, with reference to the examples of embodiment of this document, only elastomers having a molar ratio of units resulting from dienes which is less than 15% are used as diene elastomers comprising COOH functions, such as nitrile rubbers (copolymers of butadiene and acrylonitrile). The teaching of this document does not relate specifically to rubber compositions usable to form tire treads.